KR20140085396A - Manufacturing method for galvannealed steel sheet having ultra high strength - Google Patents

Abstract

The present invention provides a method for manufacturing a super high strength alloyed galvanized steel sheet, including the steps of: annealing a steel sheet including one or two or more species among C: 0.1~0.3%, Si: 0.1~1.5%, Mn: 2.0~3.0%, P: 0.001~0.10%, S: 0.010% less, Al: 0.01~0.1%, Cr: 0.3~1.0%, B: 0.0010-0.0030%, Ti: 0.01~0.1%, N: 0.001~0.01%, and Nb: 0.02~0.05%, Mo: 0.01~0.2%, V: 0.01~0.2% and W: 0.01~0.2% by weight%, and including residual Fe and inevitable impurity; cooling the annealed steel sheet; galvanizing the cooled steel sheet by dipping the cooled steel sheet into a galvanizing bath, where a temperature (T) of a cooling zone satisfies a value of 30 ~ 70 of a below relation equation 1; and performing alloying treatment of the galvanized steel sheet through induction heating. In the alloying treatment step, a threading speed (V) has a value equal to or above 70 of a below relation equation 2. Relation equation 1 : -334.1+762C +76.8Mn+43.5Cr-0.224T+0.00069T2, Relation equation 2: {17160-(156*value of relation equation 1)}/threading speed(V)(mpm).

Description

TECHNICAL FIELD [0001] The present invention relates to a galvanized steel sheet for a galvanized steel sheet,

The present invention relates to a method for manufacturing an ultra-high strength alloyed hot-dip galvanized steel sheet which can be used for a steel sheet for automobiles.

In recent years, steel plates for automobiles have been increasing the adoption of ultra-high strength steels to regulate fuel consumption for global environmental preservation and to ensure crash stability of passengers. In order to manufacture such a high-strength steel, it is not easy to secure sufficient strength and ductility by using a steel material using general solidification reinforcement or a steel material using precipitation hardening.

In order to solve this problem, the proposed technique relates to a perforated reinforcing steel utilizing a perturbation structure. Such a transformer-reinforced steel may include a dual phase steel (hereinafter also referred to as DP steel), a composite phase steel (hereinafter also referred to as CP steel), a Transformation Induced Plasticity Steel ).

DP steel is a kind of steel in which hard martensite is dispersed finely homogeneously in soft ferrite to ensure high strength and ductility. The CP steel contains two phases or three phases of ferrite, martensite and bainite, and is a steel grade containing precipitation hardening elements such as Ti and Nb for strength improvement. The TRIP steel is a steel grade that maintains strength and ductility by causing martensite transformation when finely homogeneously dispersed austenite is processed at room temperature.

Transformational steels used as automotive high strength steels (AHSS) have a metallurgical feature of securing ultra-high strength of 1Gpa or more by forming a low temperature transformation phase during cooling through securing hardenability. For this reason, in the case of the induction heating method which is mainly used in producing the galvannealed galvanized steel sheet, a large amount of retained austenite exists at the time of alloying treatment due to the increase of the hardenability. In the case of such austenite, The lower the permeability of the steel sheet, the lower the efficiency in the alloying treatment by induction heating, or it is difficult to secure the degree of alloying of the quality level required by the customer according to the conditions.

Patent Literature 1 proposes a method of producing a DP steel having a tensile strength of 1100 MPa or more by controlling a component system, and Patent Documents 2 and 3 disclose a method of producing a super high strength steel by using tensile strength of residual austenite Strength steel sheet having a strength of 980 MPa or more. Patent Documents 4 and 5 disclose a method for producing a TRIP steel having a tensile strength of 1180 MPa or more using residual austenite. However, most of these techniques have been proposed as methods for obtaining the high strength and elongation of the final material, but these techniques mainly focus on the strength, elongation or moldability of the final material. In addition, most of them are cold-rolled steel sheets, and there is a problem that the quality of alloying is significantly deteriorated due to the lowering of the permeability of the steel sheet due to retained austenite which may occur during the manufacture of the galvannealed galvanized steel sheets.

Japanese Patent Application Laid-Open No. 2008-304626 Japanese Patent Application Laid-Open No. 2008-068058 Japanese Patent Application Laid-Open No. 2007-235092 Japanese Patent Application Laid-Open No. 2003-092208 Japanese Patent Application Laid-Open No. 2004-087296

An object of the present invention is to provide a method for manufacturing an ultra-high strength alloyed hot-dip galvanized steel sheet having a tensile strength of 1 GPa or more and excellent alloying property.

The present invention relates to a ferritic stainless steel comprising 0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.01 to 0.10% 0.001-0.0030%, Ti: 0.01-0.1%, N: 0.001-0.01%, Nb: 0.02-0.05%, Mo: 0.01-0.2% W: 0.01 to 0.2%, and comprising the remainder Fe and unavoidable impurities; cooling the annealed steel sheet; and cooling the cooled steel sheet at a temperature (for example, T) is set so that the value of the following relational expression 1 satisfies the following relationship: 30 to 70, the steel sheet is dipped in a zinc plating bath and galvanized, and the galvanized steel sheet is subjected to an alloying heat treatment by induction heating In the alloying heat treatment step, the passing speed V is achieved by a method of producing an ultra-high strength alloyed hot-dip galvanized steel sheet having a value of the following relational expression 2 of 70 or more.

Relation 1: -334.1 + 762C + 76.8Mn + 43.5Cr-0.224T + 0.00069T ²

Relation 2: {17160 - (156 * value of relation 1)} / flow rate (V) (mpm)

Satisfies the relation formula 1 with respect to the component and the retained austenite fraction according to the fast cooling and cooling temperature and satisfies the relational expression (2) with respect to the alloying temperature rise temperature, it is possible to obtain a super high strength steel having excellent alloying characteristic and tensile strength of 1 GPa or more The galvannealed galvanized steel sheet can be easily produced.

The inventors of the present invention have conducted intensive studies to obtain an alloyed hot-dip galvanized steel sheet excellent in alloying characteristics while securing ultra-high strength. As a result, it has been found that the component system and the manufacturing conditions are appropriately controlled, and in particular, It has been confirmed that an extremely high strength alloyed hot-dip galvanized steel sheet excellent in alloying characteristics can be manufactured by suitably using the relational expressions relating to the retained austenite fraction and the temperature elevation temperature in the alloying treatment by using the large temperature and the passing speed, Leading to the present invention.

Hereinafter, a method of manufacturing an ultra high strength alloyed hot dip galvanized steel sheet according to one aspect of the present invention will be described in detail.

Carbon (C): 0.1 to 0.3 wt%

C is an important element added to securing the strength in the metamorphic steel. When the content of C is less than 0.1% by weight, the above-mentioned effect can not be ensured and it is difficult to ensure a tensile strength of 1 GPa or more as intended by the present invention. On the other hand, when the content of C is more than 0.3% by weight, ductility, bending workability, and weldability of the steel sheet are poor, which is difficult to apply to automotive steel sheets. Therefore, the content of C is preferably controlled to 0.1 to 0.3 wt%.

Silicon (Si): 0.1 to 1.5 wt%

Si is an element capable of improving the strength and elongation of a steel material. When the content of Si is less than 0.1% by weight, the above-mentioned effect can not be ensured. On the other hand, when the content exceeds 1.5% by weight, not only surface scale defects are caused in relation to the surface quality, but also oxides which cause unplated plated steel sheets are formed on the surface, and surface defects such as unplated and plated- ≪ / RTI > Therefore, the Si content is preferably controlled to 0.1 to 1.5 wt%.

Manganese (Mn): 2.0 to 3.0 wt%

Mn is an element that can play a large role in enhancing solubility in the presence of steel. When the content of Mn is less than 2.0% by weight, it is difficult to secure the intended strength in the present invention. On the other hand, when the content of Mn is more than 3.0% by weight, there is a high possibility that problems such as weldability and cold rolling load increase are caused, and also surface defects of the coated steel sheet can be caused by formation of coarse annealing agglomerates . Therefore, the content of Mn is preferably controlled to 2.0 to 3.0 wt%.

Phosphorus (P): 0.001 to 0.10 wt%

P is an element that can play a role in strengthening the steel sheet. If the content of P is less than 0.001% by weight, the effect to be described can not be obtained and the production cost may be a problem. On the other hand. If the content exceeds 0.10% by weight, press formability may deteriorate and brittleness of steel may be generated. Therefore, the content of P is preferably controlled to 0.001 to 0.10 wt%.

Sulfur (S): 0.010 wt% or less

S is an impurity inevitably contained and is an element which inhibits the ductility and weldability of the steel sheet. In the present invention, it is preferable to suppress the content to the maximum. In theory, it is advantageous to limit the content of S to 0%, but it is inevitably contained inevitably in the manufacturing process. Therefore, it is important to manage the upper limit, and in the present invention, the upper limit of the S content is preferably controlled to 0.010 wt%.

Aluminum (Al): 0.01 to 0.1 wt%

Al is an element effective to combine with oxygen in steel to deoxidize and distribute C in ferrite to austenite like Si to improve the hardenability of martensite. When the content of Al is less than 0.01% by weight, it is difficult to secure the above-mentioned effect. On the other hand, when the content of Al exceeds 0.1 wt%, the slab surface quality is lowered and the manufacturing cost is increased. Therefore, the content of Al is preferably controlled to 0.01 to 0.1 wt%.

Cr (Cr): 0.3 to 1.0 wt%

Cr is an element added to improve the hardenability of a steel and ensure strength. In particular, the present invention serves as an element for inducing bainite formation through ferrite transformation delay. When the content of Cr is less than 0.3% by weight, the above-mentioned effect can not be exhibited. On the other hand, when the content exceeds 1.0% by weight, the effect is saturated and the cold rolling load is increased, and the manufacturing cost is greatly increased. Therefore, the content of Cr is preferably controlled to 0.3 to 1.0% by weight.

Boron (B): 0.0010 to 0.0030 wt%

B is a component that delays the transformation of austenite into pearlite during cooling during annealing, and is an element that inhibits ferrite formation and promotes the formation of bainite. When the content of B is less than 0.0010 wt%, the above-mentioned effect can not be exhibited. On the other hand, if the content is more than 0.0030 wt%, not only the effect is saturated due to grain boundary segregation of B, but also formation of excessive surface contaminants can lead to plating defects. Therefore, the content of B is preferably controlled to 0.0010 to 0.0030% by weight.

Titanium (Ti): 0.01 to 0.1 wt%

Ti is an element added for increasing the strength of the steel sheet and for scavenging N present in the steel. When the content of Ti is less than 0.01% by weight, it is difficult to secure such effect. On the other hand, if the content of Ti exceeds 0.1 wt%, process defects such as clogging of the nozzle during the continuous casting process may be caused. Therefore, the content of Ti is preferably controlled to 0.01 to 0.1 wt%.

Nitrogen (N): 0.001 to 0.01 wt%

N is a solid solution strengthening element capable of raising the strength of a steel sheet, and is an element generally incorporated from the atmosphere. The content should be controlled by the steelmaking process degassing process. If the content of N is less than 0.001% by weight, excessive degassing treatment is required to increase the production cost. If it exceeds 0.01% by weight, high temperature ductility is deteriorated due to excessive formation of precipitates such as AlN and TiN. Therefore, the content of N is preferably controlled to 0.001 to 0.01% by weight.

The steel sheet as one aspect of the present invention includes the above-mentioned component system, and may include one or more of Nb, Mo, V, and W.

Niobium (Nb): 0.02 to 0.05 wt%

Nb is an element added for increasing the strength of the steel sheet and for refining the crystal grains. When the content of Nb is less than 0.02% by weight, it is difficult to secure the above-mentioned effect. On the other hand, when the content of Nb is more than 0.05% by weight, the production cost is increased and the bending workability and ductility can be lowered due to excessive precipitates. Therefore, the content of Nb is preferably controlled to 0.02 to 0.05 wt%.

Molybdenum (Mo), vanadium (V) and tungsten (W): 0.01 to 0.2 wt%

When Mo is less than 0.01% by weight, it is difficult to obtain an effect of increasing the strength and grain refinement. When the Mo content exceeds 0.2% by weight, the production cost is excessively increased . Therefore, the contents of Mo, V and W are preferably controlled to 0.01 to 0.2 wt%, respectively.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of manufacturing.

In addition, it is preferable that Ti and N satisfy 3.4? Ti / N. Ti in steel is an element added for increasing the strength of a steel sheet and for scavenging N present in steel, and its content preferably satisfies Ti / N = 3.4. When the Ti / N ratio is less than 3.4, the Ti addition amount is insufficient compared to the amount of dissolved N, and the strength increase effect due to the addition of B is decreased due to the formation of BN due to the residual N, and the strength may be lowered. In addition, the upper limit of the Ti / N ratio is not particularly limited, but the upper limit may be controlled to 10 in consideration of the possibility of denitrification treatment cost and clogging of nozzles in the performance process.

Nb, Mo, V and W preferably satisfy the relation of 0.02 Nb + 0.2 (Mo + V + W), but if it is less than 0.02, it is difficult to expect grain refinement and precipitation strengthening effect. The upper limit is not particularly limited, but the upper limit can be controlled to 0.05 in consideration of the manufacturing cost in terms of the effect.

The steel sheet satisfying the above-mentioned component system is annealed. When the annealing temperature is lower than 770 占 폚, the ferrite structure fraction exceeds 40%, making it difficult to secure the strength of the steel sheet, and the bending workability is lowered. On the other hand, if the annealing temperature exceeds 850 DEG C, the bending workability of the steel sheet is improved, but the amount of surface enrichment such as Si, Mn, B generated at the high temperature annealing is greatly increased, there is a problem. Therefore, the annealing step is preferably performed at 770 to 850 ° C.

Further, it is preferable that the annealing step is carried out under an atmospheric condition composed of a hydrogen concentration of 5 to 50% and the balance of nitrogen. When the hydrogen concentration is less than 5%, surface hydrates such as Si, Mn, and B in the steel having large oxygen affinity are easily generated, which may cause dent and plating defects. On the other hand, when the hydrogen concentration exceeds 50%, the increase in the above-mentioned effect is insignificant compared with the manufacturing cost. Nitrogen used as a remainder material prevents formation of a surface agglomeration of a steel sheet and is low in manufacturing cost and can be suitably used as an atmospheric gas.

It is preferable to cool the annealed steel sheet. It is preferable that the temperature (T) of the cooling zone is set so that the value of the following relational expression 1 satisfies 30 to 70.

Relation 1: -334.1 + 762C + 76.8Mn + 43.5Cr-0.224T + 0.00069T ²

The above relational expression 1 is an experimentally derived amount of retained austenite according to the cooling zone (quenching zone) temperature which determines C, M, Cr and the initiation time of bainite transformation which greatly affect the hardenability in the component system. The amount was calculated by calculating the amount of martensite present by annealing the cold rolled specimens according to the composition and quenching immediately before the alloying process. When the value of the above formula is less than 30, the amount of residual austenite is small and the amount of finally present martensite decreases, so that the target tensile strength of 1 GPa or more can not be secured in the present invention. The strength can be secured by increasing the amount of retained austenite. However, when the alloy is subjected to alloying treatment, the alloying degree of 8% or more can not be obtained due to the decrease of permeability due to the high retained austenite fraction during induction heating. Generally, the degree of alloying of the galvannealed galvanized steel sheet is expressed by the amount (%) of Fe present in the plated layer, and usually the degree of alloying exhibits excellent plating layer characteristics in the range of 8 to 14%.

It is preferable that the cooling is carried out at a rate of 100 to 600 ° C / min. When the cooling rate is less than 100 ° C / min, the strength intended for the present invention can not be secured due to the formation of ferrite and pearlite. On the other hand, when the cooling rate exceeds 600 캜 / min, the elongation rate is lowered due to the formation of an excessive hard phase, and problems such as shape defects may occur. However, it is preferable that the cooling stop temperature is controlled in the range of the martensitic transformation starting temperature (Ms) to the bainite transformation starting temperature (Bs). Through the cooling, an appropriate range of microstructure fraction can be secured.

It is also preferable that the cooled steel sheet is dipped in a zinc plating bath to be galvanized. At this time, the temperature of the zinc plating bath is preferably 480 to 520 ° C. If the temperature of the plating bath is less than 480 DEG C, the formation of the alloying inhibition layer may be insufficient and the plating may be peeled off. If the temperature exceeds 520 DEG C, a problem arises that the generation of dross increases.

The galvanized steel sheet is preferably subjected to alloying heat treatment through induction heating. It is preferable that the passing plate speed (V) during the alloying heat treatment is 70 or more in the following formula (2).

Relation 2: {17160 - (156 * value of relation 1)} / flow rate (V) (mpm)

When the value of the relational expression 2 is less than 70, sufficient alloying does not occur during the alloying process, and it is difficult to obtain a degree of alloying of the steel sheet of 8% or more. When the degree of alloying is less than 8%, not only the weldability, which is an advantage of the galvannealed galvanized steel sheet, deteriorates but also causes a plating surface defect due to a decrease in flaking resistance. The apparatus for performing the induction heating in the present invention is not particularly limited, but it is preferable to apply induction heating which satisfies the frequency range of 0.5 to 300 KHz and the usable output range of 2000 to 6000 kW.

Hereinafter, the ultrahigh strength alloyed hot-dip galvanized steel sheet according to another aspect of the present invention will be described in detail.

The galvannealed steel sheet satisfying the above-mentioned component system can secure a tensile strength of 1 GPa or more. The microstructure of the steel sheet preferably contains 40 to 70% by area of bainite and residual ferrite and martensite. When the content of bainite is less than 40% by area, the bending workability decreases. When the bainite content exceeds 70% by area, it is difficult to realize a high tensile strength of 1 GPa or more. The fraction of the ferrite and martensite structure is not particularly limited. However, in order to secure excellent strength, ductility and bending workability, it is preferable that the ferrite structure and the martensite structure have a range of 10 to 40% by area and 15 to 30% Do.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

(Example)

Slabs satisfying the component system as shown in the following Table 1 were prepared, hot and cold-rolled, and then annealed at a temperature of 780 to 850 ° C and cooled to a temperature higher than the martensite start temperature, The temperature of the zone (fast cooling zone) was controlled as shown in Table 2 below. Galvanized by immersing in a zinc plating bath at 500 DEG C and then subjected to alloying heat treatment through induction heating according to the conditions shown in Table 2 below.

Table 2 shows the values of the relational expressions 1 and 2 and the temperature of the cooling zone, the sheet passing speed during the alloying heat treatment, the inlet temperature, the outlet temperature, the temperature rise temperature and the residual austenite fraction of the alloying heat treatment furnace. The yield strength (YS), the tensile strength (TS), the elongation (EL) and the degree of alloying of the inventive example and the example were measured and are shown together in the following Table 2.

(However, the unit of each element is% by weight.)

As shown in Table 2, Inventive Examples 1 to 9 are examples that satisfy the relational expressions 1 and 2 of the constituent system controlled by the present invention, and show that alloying degree in the alloying treatment by induction heating is 8 to 14% And it was confirmed that the tensile strength of the final galvannealed galvanized steel sheet satisfied the target of 1 GPa or more, which is the aim of the present invention.

On the other hand, in Comparative Examples 1 to 5, the value of the relational expression 1 regarding the amount of retained austenite limited in the present invention is more than 70, the induction heating efficiency is lowered due to the decrease of the permeability according to the high retained austenite fraction, , It was impossible to secure the alloying degree to 8% or more. Further, the comparative steels 6 and 7 satisfy the range of the relational expression 1 satisfying the range controlled by the present invention, so that the retained austenite fraction is appropriate, but the value of the relational expression 2 is less than 70 due to the high throughput rate, It was not possible to secure the figure. In addition, in Comparative Examples 8 to 11, the martensite fraction fell outside the range defined by the present invention, and the value of the relational expression 1 was less than 30, so that the desired tensile strength of 1 GPa or more could not be secured in the present invention.

Claims (8)

0.1 to 0.3% of C, 0.1 to 1.5% of Si, 2.0 to 3.0% of Mn, 0.001 to 0.10% of P, 0.010% or less of S, 0.01 to 0.1% of Al, 0.3 to 1.0 0.01 to 0.2% of Mo, 0.01 to 0.2% of V, 0.01 to 0.2% of W, 0.01 to 0.2% of Mo, 0.001 to 0.0030% of B, 0.01 to 0.1% of Ti and 0.001 to 0.01% of N, To 0.2%, and the remainder being Fe and unavoidable impurities;
Cooling the annealed steel sheet;
In the cooling step, the temperature (T) of the cooling zone is determined so that the value of the following relational expression 1 satisfies 30 to 70,
Dipping the cooled steel sheet in a zinc plating bath to zincate the steel sheet;
And subjecting the galvanized steel sheet to an alloy heat treatment by induction heating,
In the alloying heat treatment step, the passing speed (V) is a value of the following relational expression (2): 70 or more.
Relation 1: -334.1 + 762C + 76.8Mn + 43.5Cr-0.224T + 0.00069T ²
Relation 2: {17160 - (156 * value of relation 1)} / flow rate (V) (mpm)
The method according to claim 1,
Wherein Ti and N satisfy 3.4 = Ti / N.
The method according to claim 1,
Wherein the Nb, Mo, V and W satisfy 0.02 = Nb + 0.2 (Mo + V + W).
The method according to claim 1,
Wherein the annealing step is performed at 770 to 850 ° C.
The method according to claim 1,
Wherein the annealing step is performed under a condition that the hydrogen concentration is 5 to 50 vol% and the remainder is nitrogen.
The method according to claim 1,
Wherein the cooling step is performed at a cooling rate of 100 to 600 ° C / min.
The method according to claim 1,
Wherein said cooling step stops cooling at a martensitic transformation starting temperature (Ms) to a bainite transformation starting temperature (Bs).
The method according to claim 1,
Wherein the zinc plating bath has a temperature of 480 to 520 ° C.